Today’s Machining World Archive: October 2006, Vol. 2, Issue 10
From Ore to Insert:
Don Graham, turning products manager at Seco Tools, Inc. (formerly Seco-Carboloy), told the story of how tungsten ore is processed into tungsten carbide, which then becomes a state of the art cutting tool insert. Additional information provided by the web sites of the following organizations: the American Chemical Society’s Chemical & Engineering News, American Machinist magazine, International Tungsten Industry Association, Kennametal and Seco-Carboloy.
Tungsten carbide, often called simply “carbide,” is a familiar material around the shop. This compound of tungsten and carbon has revolutionized the metal-cutting world over the decades, enabling increased speeds and feeds and providing longer tool life. Tungsten carbide was fi rst investigated as a cutting tool material in 1925, by Dr. Samuel Hoyt, a scientist at General Electric’s Lamp Department. Later GE opened the Carboloy division to produce tungsten carbide cutting tools. In the late 1930s, Philip M. McKenna, founder of Kennametal, discovered that adding a titanium compound to the mix made the tools work better at higher speeds. This began the march toward today’s lightning-fast cutting speeds.
“Cemented tungsten carbide,” the material that makes up the tools and inserts, is actually grains of tungsten carbide, along with particles of other materials, cemented together using the metal cobalt as a binder.
It starts in the ground
There are several tungsten ores that can be mined and refined into tungsten or made into tungsten carbide. Wolframite is the best known. The ore is crushed, heated and treated with chemicals. The result: tungsten oxide.
Then, the fine particles of tungsten oxide are carburized, turning them into tungsten carbide. In one method, the tungsten oxide is mixed with graphite (carbon). This mixture is heated to over 1200˚ C (2200˚ F) and a chemical reaction occurs that removes the oxygen from the oxide and combines the carbon with the tungsten to yield tungsten carbide.
Grain size is key
The size of the carbide grains determines the mechanical properties of the final product. The size of the grains will depend on the size of the tungsten oxide particles, and how long and at what temperature the oxide/carbon mixture is processed.
The tungsten carbide particles are a fraction of the size of a grain of sand. They are likely to range in size from half a micron, to as large as 10 microns. A series of sieves sorts out the different grain sizes: less than one micron, one and one half microns, and so forth.
At this point, the tungsten carbide is ready for blending into “grade powder.” In the tungsten carbide industry, one speaks of grades rather than alloys, but they mean the same thing.
The tungsten carbide goes into a mixing vessel with the other components of the grade. Powdered cobalt metal will act as the “glue” to hold the material together. Other materials, such as titanium carbide, tantalum carbide and niobium carbide are added to improve the properties of the material when cutting. Without these additives, when cutting ferrous materials, the tungsten carbide tool may experience a chemical reaction between the tool and the chips of the work piece that leaves craters in the tool, especially at high cutting speeds.
Mix it up
All these ingredients are blended with a liquid such as alcohol or hexane and placed in a mixing vessel, often a rotating drum called a ball mill. In addition to the grade ingredients, cemented balls 1/4″ to 5/8″ in diameter are added, to help the process of adhering the cobalt to the carbide grains. A ball mill may be as small as five inches in diameter by five inches long, or as large as a 55-gallon drum.
When the mixing is complete, the liquid must be removed. This typically happens in a spray dryer, which looks like a stainless steel silo. An inert drying gas, nitrogen or argon, is blown from the bottom up. When all the liquid is removed, the remaining dry material is “grade powder,” which looks like sand.
For cutter inserts, the grade powder goes into insert shaped molds specially designed to allow for the shrinkage that will happen later on in the process. The powder is compressed into the molds, in a process similar to how pharmaceutical tablets are formed.
Taking the heat
After pressing, the form looks like oversized inserts and is fairly delicate. They are removed from the molds and placed on graphite or molybdenum trays, and go into a sintering furnace where they are heated in a low-pressure hydrogen atmosphere to 1100-1300˚ C (about 2000- 2400˚ F). The cobalt melts, and the insert consolidates into a solid, smaller size.
After the inserts are removed from the furnace and cooled, they are dense and hard. After a quality control check, the inserts are usually ground or honed to achieve the correct dimensions and cutting edge. Honing to a radius of 0.001″ is typical, though some parts receive a cutting-edge radius of half a thousandth or as large as 0.002″, and some are left “dead sharp,” as sintered.
Some types and designs of inserts come out of the sintering furnace in their final shape and in-spec, with the correct edge, and don’t need grinding or other operations.
The process for manufacturing blanks for solid carbide tools is very similar. The grade powder is pressed to shape and then sintered. The blank or stock may be ground to size afterward before shipping to the customer, who will form it by grinding or perhaps EDM.
Inserts bound for most non-ferrous applications may be ready to package and ship at this point. Those destined for cutting ferrous metals, high temperature alloys or titanium, will need to be coated.
Tungsten, in its elemental form, is a silver colored metal. Its atomic number is 74 and its average atomic mass is 183.85. One of the densest metals, it is more than twice as dense as steel. And it has the highest melting temperature of any metal: 3422˚ C (over 6000˚ F).
We call it “tungsten,” which means “heavy stone” in Swedish. So why is its chemical symbol “W”? That comes from its other name, wolfram. Legend has it that back in the 1600s, miners noticed that a particular ore (which turned out to contain tungsten) interfered with smelting tin; it seemed to eat up the tin as a wolf devours its prey.
Two common ores of tungsten, wolframite and scheelite, were discovered in Sweden in the 1700s, and in 1783 the metal was isolated by two Spaniards, who named it wolfram.
According to the International Tungsten Industry Association, the majority of tungsten reserves are in China, and currently about 80 percent of tungsten is mined there. In the last couple of years, the price of tungsten ore has risen sharply. Perhaps as a result, recycling of tungsten, including tungsten carbide, is on the rise.
Coatings finish the job
To prolong tool life under challenging cutting conditions, many types and combinations of coatings have been developed. They can be applied in two ways: by chemical vapor deposition (CVD) or physical vapor deposition (PVD). Both types are applied in furnaces.
Chemical vapor deposition
For CVD, the coating is usually 5- 20 microns thick. Milling and drilling inserts usually receive 5–8 icrons, as these operations require better surface finish, and they encounter more impact, so they require greater edge toughness. For turning applications, the coatings tend to be in the range of 8–20 microns. In turning, heat and abrasion tend to be more of a concern.
Most, but not all, CVD coatings are made up of multiple layers, usually three distinct layers.
Each company has its own “recipe” for coatings. Here is a typical scheme, building up three layers.
Here is a typical scheme, building up three layers.
• one layer of titanium carbo-nitride for hardness and abrasion resistance
• one layer of aluminum oxide, which retains hardness at higher temperatures and is chemically very stable
• one layer of titanium nitride, which prevents metal buildup from fragments of the workpiece welding to the tool. This coating is gold-colored and makes it easy to observe wear of the edge. To apply a CVD coating, the parts are placed on trays and sealed in a furnace. The furnace is drawn down to a vacuum.
For each layer, the appropriate gases are introduced into the furnace, such as hydrogen, titanium tetrachloride, methane, nitrogen, aluminum chloride. A chemical reaction occurs, depositing the layer of coating on the inserts.
The aluminum oxide provides thermal protection, keeping heat out of the body of the insert, important for high speed applications. For low speed applications, an insert may not need an aluminum oxide layer.
Physical vapor deposition
PVD coatings are typically about 2-4 microns thick. Different manufacturers use different numbers of layers. These PVD coatings are well-suited to applications cutting high temperature, nickel-based, cobalt-based or titanium-based materials, and sometimes steel and stainless steel.
Titanium carbo-nitride, titanium nitride and titanium aluminum nitride are widely used as PVD coatings. The latter is the hardest and most chemically stable PVD coating.
The inserts are mounted on racks so they are separated from each other. Each rack rotates and the whole assembly of racks revolves within the furnace, so every surface of the inserts is exposed to the deposition process. The furnace is evacuated.
Strong negative charge is applied to the inserts. A piece of titanium, or titanium and aluminum is installed on the wall or floor of the furnace. The metal is vaporized by either an electric arc or an electron beam, liberating the positively charged metal ions. These ions are attracted to the negatively charged inserts. Nitrogen and methane are added as appropriate, to achieve the different types of coatings.
When the inserts are removed from the furnace they may be ground again, or directly packaged and shipped.
Tool manufacturers are meeting the pressures for ever increasing feeds and speeds, and the need for longer tool life and lower costs, by continually improving the designs of tungsten carbide cutting tools and developing better and better coating technologies.